The present invention relates to a governing mode switching apparatus for steam turbnes and, more particularly, to a governing mode change-over apparatus designed to minimize the load change on the steam turbine when switching.
Conventionally, the speed of a steam turbine is controlled by two modes: the throttle governing mode in which a plurality of steam control valves (flow rate control valves) are simultaneously opened, and the nozzle governing mode in which a plurality of control valves are opened successively one by one. It is well known that each governing mode has its advantages and disadvantages, depending on the valve opening characteristics of its own. For instance, in the throttle governing mode, steam is uniformly supplied to the whole portion of the steam turbine because the steam control valves are simultaneously opened at the same opening rate, so that the turbine temperature is uniformly raised thus minimizing thermal stress. In this case, however, the loss of steam energy is inevitably caused during throttling, because the steam control valves are only gradually opened, so that the efficiency is lowered in the partial load operation of the steam turbine. On the other hand, in the nozzle governing mode, the steam control valves are individually sequentially opened, so that the loss of steam energy is minimized because each valve can open quickly. In this case, however, a large temperature difference is caused between the portion of the turbine downstream of the valve which has been already opened and the portion downstream of the valve which is still closed.
In the case of a power generating steam turbine having high output, the selection between these two types of governing modes has been determined mainly from the viewpoint of operation characteristics required for these turbines. For instance, with a power generating steam turbine intended for peak load, i.e., a medium size electric power plant which is started early in the morning and stopped late at night so as to meet the daytime peak load, the throttle governing mode is preferably used because it minimizes thermal stress despite the frequent starting and stopping of the turbine. On the other hand, with steam turbines which are intended for base load, i.e., power generating turbines which are operated continuously to bear the base load throughout a year; hence, are stopped, and started once or so a year, the nozzle governing mode is preferably used because it can ensure a high thermal efficiency over a wide load range and because the problem concerning the start-up is not so severe.
Thus, conventional turbine control systems have selectively adopted either one of these modes for coping with the demand.
A turbine control system has been proposed in, for example, Japanese patent Publication No. 7123/1971, wherein the speed is governed in the throttle speed governing mode when the load is lower than a predetermined level, e.g., 50%, whereas, when this predetermined load level is exceeded, the speed is governed in the nozzle governing mode. This turbine control system, however, involves a problem such that, for instance, the nozzle governing mode cannot be used at 30% load, while, at 70% load, the throttle governing mode cannot be used, because the level of the load at which the change-over between two modes is conducted is fixed.
In a steam turbine with a nozzle governing system, severe erosion is caused in a specific nozzle because the steam is supplied only through that nozzle during partial load. This erosion is made worse by thermal stress.
Nozzle erosion is primarily caused by boiler scale which is most severe immediately after plant has been started up after being shut down for a while. Erosion decreases rapidly after this. With this knowledge, it has been proposed to use a throttle governing mode during a start-up after a stop and switch it into a nozzle governing mode after scale and other foreign matter have been disposed off, regardless of the load level, thereby affording high thermal efficiency. For this purpose a mechanical type controller has been proposed to control a turbine. In this proposed arrangement, two cams having different countours corresponding to the throttle governing mode and the nozzle governing mode are used and the switching between these two modes is conducted by utilizing one of the cams as a fulcrum while the other is being used.
In this switching system, since one of the cams is a fulcrum while the other is used, there is a risk that, for example the valve subjected to load control immediately before the governing mode change-over is not operated at a predetermined load but is temporarily closed to an degree of opening less than or greater than the opening necessary for the load.
Another problem is that, since all valves are operated simultaneously actuated, the aggregate fluctuation of all valves causes a large fluctuation in the total flow rate of the steam passing through the steam control valves, resulting in a large change in the output power of the generator.
An object of the invention is to provide a governing mode switching apparatus which is capable of switching between the nozzle governing mode and the throttle governing mode, while controlling the load at a constant level, thus minimizing the change in the load during the switching of the governing mode.
In accordance with advantageous features of the present invention, at least one of the steam control valves is subjected to load control so that the load is controlled at a constant level, while the other steam control valves are operated to the desired degree of opening during the governing mode change-over.